Gas phase infrared multiple-photon dissociation spectra of methanol,

ethanol and propanol proton-bound dimers, protonated propanol and the

propanol/water proton-bound dimer

Travis D. Fridgen,*aLuke MacAleese,

bTerry B. McMahon,

cJoel Lemaire

band

Philippe Maitreb

Received 23rd November 2005, Accepted 11th January 2006

First published as an Advance Article on the web 26th January 2006

DOI: 10.1039/b516661f

The infrared multiphoton dissociation (IRMPD) spectra of three homogenous proton-bound

dimers are presented and the major features are assigned based on comparisons with the neutral

alcohol and with density functional theory calculations. As well, the IRMPD spectra of

protonated propanol and the propanol/water proton-bound dimer (or singly hydrated protonated

propanol) are presented and analysed. Two primary IRMPD photoproducts were observed for

each of the alcohol proton bound dimers and were found to vary with the frequency of the

radiation impinging upon the ions. For example, when the proton-bound dimer absorbs weakly a

larger amount of SN2 product, protonated ether and water, are observed. When the proton-bound

dimer absorbs more strongly, an increase in the simple dissociation product, protonated alcohol

and neutral alcohol, is observed. With the aid of RRKM calculations this frequency dependence

of the branching ratio is explained by assuming that photon absorption is faster than dissociation

for these species and that only a few photons extra are necessary to make the higher-energy

dissociation channel (simple cleavage) competitive with the lower energy (SN2) reaction channel.

1. Introduction

Among the many novel bonding features exhibited by gas

phase ions, strong hydrogen bonding is perhaps the most

interesting and most important. In the formation of an ion–

molecule association adduct, the presence of a centre of charge

enhances both electrostatic and polarization interactions but,

in addition, can also induce changes in the covalent structure

of the participants in the association adduct. In the case of

hydrogen bonded adducts, this change in covalent structure

frequently takes the form of a partial proton transfer in which

a proton appears to be shared between two, or more, basic

sites of the molecules involved in adduct formation. In the case

of adducts involving the same atom or molecule, the proton

may be shared equally by the participants. Thus, for example,

the protonated water dimer, H5O21, is predicted to exhibit a

structure in which the bridging proton is equidistant from the

two oxygen atoms of the participating water molecules.1 This

yields the so-called Zundel structure for the protonated water

dimer. Similarly, the bifluoride ion, FHF�, which has the

strongest known hydrogen bond, has a proton symmetrically

disposed between the two fluoride ions in a linear geometry.2

Structures such as these two give rise to a relatively broad

potential well for the proton motion which is flatter than a

more conventional parabolic potential and thus leads to an

anharmonic character of the vibrational motion associated

with the proton motion.

Very recently, the advent of the use of tunable, high-power,

infrared-output, free-electron lasers (FEL) in conjunction with

ion trap mass spectrometers (either FTICR or quadrupole)

has permitted the study of the wavelength dependence of

photo-dissociation of ion–molecule adducts which, in effect,

yields a vibrational signature of the adduct in question. In

addition OPO lasers have recently become available with

tunability through the common fingerprint range of many

molecules in the 600–2000 cm�1 range. Although the power

of these OPO lasers is usually insufficient to induce the

dissociation of strongly hydrogen bonded adducts, the use of

a ‘‘messenger’’ atom or molecule,3 weakly attached to the

species of interest, permits a vibrational signature to be

obtained through the loss of the messenger species. Both

FEL and OPO techniques have recently been applied to the

investigation of the vibrational spectrum of the H5O21 species.

The two FEL studies4,5 and the OPO investigation6 all agree

on the presence of an intense band in the 1740–1770 cm�1

range corresponding to the bending motion of the terminal

water molecules with mixing of the central O–H–O stretch.

Based on a fairly low level density functional theory (DFT)

calculation of the harmonic frequency of the central O–H–O

asymmetric stretch as 1004 cm�1, Fridgen et al.4 assigned the

relatively sharp feature in their FEL spectrum at 990 cm�1 to

this vibration. Using a more sophisticated quantum 4D model

Vener et al.7 calculated a value of 1158 cm�1 for this vibration

to which Asmis et al.5 assigned a 1317 cm�1 feature in their

FEL spectrum. The OPO-messenger technique of Headrick

aDepartment of Chemistry, Memorial University of Newfoundland,St. John’s NL, Canada A1B 3X7

bLaboratoire de Chimie Physique, UMR 8000 CNRS-Universite deParis XI, Campus d’Orsay, Batiment 350, 91405 Orsay Cedex,France

cDepartment of Chemistry, University of Waterloo, Waterloo, ON,Canada N2L 3G1

This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 955–966 | 955

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

et al.6 yields much sharper features, although admittedly

somewhat blue-shifted in the case of the asymmetric stretch

by the perturbation induced by the presence of the Ar or Ne

messenger.8 They find this band to be centered at 1080 cm�1

and estimate the blue-shift to be on the order of 60–110 cm�1

which would place the vibration for the isolated H5O21 species

at 970–1020 cm�1, in excellent agreement with the 990 cm�1

experimental assignment of Fridgen et al.4

Protonated dimers of oxygen n-donor bases in general

represent an especially interesting class of species for study.

The high electronegativity of oxygen results in substantial

hydrogen bond energies. For example, Larson andMcMahon9

have shown experimentally that symmetric proton bound

dimers of oxygen bases, no matter of what functionality, all

have hydrogen bond strengths of 30.5 � 1 kcal mol�1 and

quantum chemical calculations suggest that all have a sym-

metric, single minimum structure associated with the central

proton. With the exception of the hydrogen bonds found in

the bifluoride ion,10,11 FHF�, and the proton-bound dimer of

fluoromethane,12 (CH3F)2H1, these represent the strongest

known hydrogen bonds and are thus of considerable interest

to study as a class of bonding. Moore et al.13 were the first to

investigate the IR spectrum of proton bound dimers of an

organic oxygen base using a FEL facility. They obtained the

IR photo-dissociation spectra of the proton bound dimers of

dimethyl ether and diethyl ether from 600–1800 cm�1. In both

cases a well resolved feature was observed between 750 and

825 cm�1 which could be associated with the asymmetric

stretch mode of the O–H–O unit. Fridgen et al.14 subsequently

obtained spectra of the homogenous-proton bound dimers of

dimethyl ether, tetrahydrofuran and 1,4 dioxane. The spec-

trum for dimethyl ether was in excellent accord with that of

Moore et al.13 and they also showed that the two cyclic ethers

each exhibited well resolved bands in the same range for the

asymmetric stretch at 801 and 753 cm�1, respectively. In

addition the IRMPD spectrum of protonated diglyme was

also obtained which, in essence is an intramolecular symmetric

proton bound dimer.13 It too showed a sharp absorption

feature at 760 cm�1 associated with the asymmetric stretch

of the intramolecular bonding hydrogen. It is noteworthy that,

in the cases of all of the symmetric proton bound dimer of

ether species studied, excellent agreement was obtained be-

tween hybrid-DFT (B3LYP/6-31þG**) calculated vibrational

frequencies for the asymmetric stretching motion and those

experimentally observed.14 This was, however, not the case for

asymmetric proton bound dimers where considerable differ-

ences between calculated and observed frequencies were

found.14 This can be attributed to the sensitivity of the

calculated frequency to the exact position of the bridging

proton in the asymmetric hydrogen bond. While minor dis-

placements have little effect on the hydrogen bond energy, due

to the rather flat nature of the potential, such displacements

can, in fact, have a considerable influence on the calculated

vibrational frequency.

In the present work, the study of the vibrational signature of

proton bound dimers of oxygen n-donor bases is extended to

alcohols. These species represent an interesting case since they

are intermediates in the SN2 reaction between protonated and

neutral alcohols from which water elimination may occur to

yield protonated ether as the ionic product (see eqn (1) and

Scheme 1).

ðROHÞ2Hþ �!nhn

R2OHþ þH2O ð1Þ

Thus the dissociation of activated proton bound dimers may

proceed either via simple dissociation to yield the protonated

plus neutral alcohol (eqn (2)) or via rearrangement to the SN2

intermediate to yield protonated ether plus water. Fridgen and

McMahon have previously examined the potential energy

surfaces for these reactions, both experimentally and theore-

tically, and found that the SN2 reaction channel is favored

enthalpically, although involving a tighter, less entropically

favored, transition state.15,16 In previous IRMPD experiments

using a continuous wave CO2 laser, Bomse and Beauchamp17

observed exclusive formation of protonated ether products

from the proton bound dimers of methanol, ethanol and iso-

propanol. For n-propyl and n-butyl alcohol proton bound

dimers, however, other products were observed. In the n-

propyl case, for example, the major ionic product was proto-

nated n-propanol from simple bond cleavage (eqn (2)) with

small amounts of protonated di n-propyl ether (17%) and a

protonated propanol–water adduct (12%, eqn (3)) being

formed.

ðROHÞ2Hþ �!nhn

ROHþ2 þROH ð2Þ

ðCH3ðCH2Þ2OHÞ2Hþ �!nhn

CH3ðCH2Þ2OHþ2 ðH2OÞþ CH3CHQCH2 ð3Þ

In addition to the straightforward assignment of the vibra-

tional spectra of proton bound dimers of alcohols, the present

work shows very interesting trends in branching ratios for the

possible dissociation products. These trends provide some

considerable insight into the nature of the FEL IR photo-

dissociation process.

2. Methods

2.1 Experimental

The coupling of the FTICR and FEL as well as the method for

generating the proton bound dimers has been described in

detail previously14 and will not be repeated here. All experi-

ments were conducted at the Centre Laser Infrarouge d’Orsay

(CLIO),18 which houses a free electron laser (FEL) to which a

Scheme 1 SN2 Reaction of proton-bound methanol dimer.

956 | Phys. Chem. Chem. Phys., 2006, 8, 955–966 This journal is �c the Owner Societies 2006

Mobile Ion Cyclotron Resonance Analyser (MICRA)19 has

been coupled. IRMPD Efficiency spectra are obtained by

observing IRMPD of an ion of interest and all photoproducts

of IRMPD as a function of radiation wavelength. The

IRMPD efficiency is defined in eqn (4).

� lnIparent

Iparent þPi

IfragmentðiÞ

0@

1A ð4Þ

The FEL wavelength was scanned by B5 cm�1 steps, the laser

bandwidth is about 0.3–0.5% of the spectral wavelength. Ions

were irradiated for typically 2 s. The laser wavelength and its

bandwidth are monitored with a monochromator associated

with a spiricon multichannel detector.

2.2 Computational

Geometry optimization and frequency calculations were per-

formed using the hybrid density functional method B3LYP

and the 6-31þG** basis set within the Gaussian 0320 suite of

programs. These calculations have proven to be quite ade-

quate in predicting the infrared spectrum of the proton-bound

dimer of water4 and the complex of H5O21 with Ar,6 as well as

various ether proton-bound dimers.14 Except for protonated

propanol the infrared frequencies were used unscaled.

2.3 RRKM calculations

The rate constants for the SN2 reaction and for simple

dissociation of the proton-bound alcohol dimers, eqn (1) and

(2), respectively were calculated using RRKM theory21 given

by,

kuni ¼sh

NaðE � E0ÞrðEÞ ð5Þ

where s is the symmetry number, h is Planck’s constant. The

sum (Na) and density (r) of states were calculated using the

Beyer–Swinehart direct count algorithm, E is the internal

energy of the reacting proton-bound dimer, and E0 is the

activation energy. The vibrational wavenumbers for the pro-

ton-bound dimers of methanol, ethanol and propanol as well

as the transition states for reactions 1 and 2 used in this

RRKM modeling were taken from McCormack and Mayer.22

The activation energies used were 135, 135 and 132 kJ mol�1

for the methanol, ethanol, and propanol proton-bound di-

mers, respectively,23 for the simple dissociation reaction (eqn

(2)) and were 16.9, 16.1 and 15.7 kJ mol�1 less, respectively,

for the SN2 reactions (eqn (1)).15,16

3. Results and discussion

3.1 IRMPD of protonated alcohol dimers

Upon resonant infrared irradiation of the alcohol proton-

bound dimers with the CLIO FEL dissociation was observed.

For all three proton bound dimers the observed products

following infrared multiple photon absorption included pro-

tonated alcohol, produced by loss of neutral alcohol (eqn (2)),

as well as protonated ether formed by dehydration of the

proton-bound dimer (eqn (1)). In Fig. 1 is shown the mass

spectra recorded without (Fig. 1a) irradiating proton-bound

ethanol dimers and after irradiation of proton-bound ethanol

dimers with light centred at 760 cm�1 from the CLIO FEL

(Fig. 1b), clearly showing both products.

In previous experiments conducted by irradiating proton-

bound alcohol dimers with a low-powered (15–50 W cm�2)

CO2 laser (10.6 mm line), Bomse and Beauchamp17 found that

only when the alkyl group (R) is n-propyl, i-propyl, and

s-butyl, were multiple products observed. When R is methyl or

ethyl, the only products were the protonated ethers, eqn (1), as

discussed above. For the proton-bound dimer of n-propanol

the major ionic product was protonated propanol at 71% (eqn

(2)). That multiple products are also observed for R = CH3

and CH2CH3 in the present IRMPD experiments certainly

deserves some discussion.

3.1.1 IRMPD of methanol proton-bound dimer. As already

stated, for the present experiments both protonated ether and

protonated alcohol were observed for the proton-bound di-

mers of methanol, ethanol and propanol. In the case of

methanol, the major dissociation product was observed at

m/z 47, protonated dimethyl ether. Fig. 2 contains a plot of the

product signal intensities against the wavenumber of irradia-

tion as well as the IRMPD efficiency spectrum (or infrared

spectrum) of protonated methanol (to be discussed in Section

3.2.1). It is interesting to compare these two plots. It should be

pointed out first, that over the entire range of the infrared

covered in these experiments, the proton-bound dimer dis-

sociates to a small extent indicating that there is at least a small

amount of absorption by the proton-bound dimer at all

frequencies. Both products increase in intensity upon strong

absorption as might be expected for two competing dissocia-

tion pathways. What may not be quite apparent from Fig. 2, is

Fig. 1 Mass spectra recorded after (a) no irradiation and (b) irradia-

tion of ethanol proton-bound dimers with the CLIO FEL tuned to

produce light with a wavenumber value of 760 (�10) cm�1.

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that during a strong absorption of the CLIO FEL, m/z 33

increases by a greater amount than m/z 47. This is more clearly

shown in Fig. 3 which is a plot of intensity ratios (m/z 47 : m/z

33) against the wavenumber of the light impinging on the ions.

Upon irradiation of the ions where absorption is the greatest

(858 cm�1) the ratio of products is smaller, meaning that more

m/z 33 compared to m/z 47 is produced, however, the ratio

does stay above 1. The ratio increases for weaker absorptions

(more m/z 47).

Another possibility for the higher energy dissociation (eqn

(2)) is subsequent IRMPD of m/z 47, eqn (6). It is necessary to

rule out this other pathway for formation of m/z 33 prior to

discussing the wavenumber-dependent branching ratio. The

reaction in eqn (6) would involve a rearrangement concomi-

tant with or prior to dissociation, and would be expected to

have a significant barrier.

ðCH3Þ2OHþ �!nhn CH3OH þ2 þ CH2 ð6Þ

This reaction can be ruled out since simple cleavage, eqn (7), is

more thermodynamically favoured by 65 kJ mol�1 and would

not have a significant central energy barrier. If m/z 47 were

subsequently undergoing IRMPD, CH31 (m/z 15) would be

the expected product which is not observed.

ðCH3Þ2OHþ �!nhn CH3OHþ CH þ3 ð7Þ

The most convincing evidence for m/z 33 being a primary

photoproduct of methanol proton-bound dimer IRMPD, and

not sequential IRMPD of protonated dimethyl ether, is that

no m/z 33 was observed during the IRMPD of the dimethyl

ether proton-bound dimer14 which produces m/z 47 (proto-

nated dimethyl ether) exclusively.

In Fig. 4, the intensity ratio is plotted against the IRMPD

efficiency which shows that at higher IRMPD efficiency, the

ratio is low and vice versa. In the Bomse and Beauchamp

experiments,17 the 10.6 mm line of the CO2 laser (B945 cm�1)

is not where the methanol proton-bound dimer is very strongly

absorbing, but in this region, in the present experiments, the

m/z 47 : m/z 33 ratio is between 3 and 4. That m/z 33 is still

observed in the present experiments likely means that the

proton-bound dimers are absorbing more photons than ne-

cessary to dissociate by the lowest-energy route (eqn (1)) in the

present experiments. The power density of the CLIO FEL has

been measured to be on the order of 106 W cm�2 over the

infrared range studied here, significantly higher than for a CO2

laser. The enthalpy barrier for the reaction in eqn (1) was

experimentally determined to be B17 kJ mol�1 below that for

the simple dissociation reaction (eqn (2)).15 This energy is less

than that contained in two infrared photons (860 cm�1 E 10.3

kJ mol�1) in the range of the present experiments. The

observation of more of the higher energy dissociation product

(protonated methanol) at wavenumber positions where the

proton-bound dimer absorbs more strongly suggests that the

proton-bound dimer, upon irradiation at these wavenumbers,

has an internal energy which is high enough that the dissocia-

tion pathway (eqn (2)) has a higher rate constant than the SN2

pathway (eqn (1)). Therefore, when the proton-bound dimer

Fig. 2 Plot of the product ion intensities (m/z 47 circles and m/z 33

squares) vs. wavenumber (ordinate on the left) as well as the IRMPD

spectrum of the methanol proton-bound dimer (ordinate on the right).

The grey bar indicates the magnitude of background signal intensity

for the mass spectra.

Fig. 3 Plot of the ratio of product ion intensities (m/z 47 : m/z 33) vs.

radiation wavenumber. Also shown is the IRMPD efficiency spectrum

for protonated methanol. The arrows point toward the ordinate for

each plot. The stars at the bottom of the spectra are the points selected

for the filled symbols in Fig. 4.

Fig. 4 Plot of the product ion intensity ratio (m/z 47 : m/z 33) vs.

IRMPD efficiency. The filled circles are the intensities and efficiencies

for the band maxima.

958 | Phys. Chem. Chem. Phys., 2006, 8, 955–966 This journal is �c the Owner Societies 2006

absorbs strongly, absorption competes effectively with the SN2

reaction pathway (eqn (1)), thereby increasing the internal

energy of the proton-bound dimer, making the simple disso-

ciation pathway (eqn (2)) more efficient. In Fig. 5 is plotted the

RRKM calculated rate constants for the reactions in both eqn

(1) and (2) against the number of photons absorbed (bottom

axis) or against the internal energy content (top axis) for the

methanol proton-bound dimer. It is shown from this figure

that after absorption of about 12 photons (at 1000 cm�1) H2O

elimination (eqn (1)) is possible. Above 1.94 eV internal energy

(or after absorption of B16 photons) the simple dissociation

mechanism (eqn (2)) becomes the dominant dissociation path-

way. This is consistent with our explanation if the rate of

photon absorption at the wavelength positions where the

proton-bound dimer absorbs very strongly is greater than

the rate of reaction in eqn (1).

For completion it should be noted that protonated dimethyl

ether was found to be only a minor dissociation product in

CO2 laser IRMPD experiments.24 Double-resonance experi-

ments showed that it was not formed via the m/z 33 IRMPD

product reacting with background methanol. We can offer no

explanation at this time for the fact that m/z 33 is the

dominant IRMPD product in Peiris et al.24 experiments.

3.1.2. IRMPD of ethanol and propanol proton-bound di-

mers. IRMPD of the proton-bound dimer of ethanol produces

predominantly m/z 47 (protonated ethanol) via eqn (2), with

some m/z 75 (protonated diethyl ether). Here as well, there is

evidence for some variation in the m/z 75 : m/z 47 ratio as

shown in Fig. 6. Note that the ratio only varies by a factor of

2.5 whereas for the proton-bound dimer of methanol, the ratio

varies by a factor of about 5. In the case of the ethanol proton-

bound dimer some IRMPD of the m/z 75 photoproduct,

cannot be completely excluded. Tonner and McMahon25

observed almost complete dissociation of protonated dimethyl

ether (eqn (8)) after 30 s irradiation with a CO2 laser operating

at 10.6 mm (943 cm�1) and 10 W cm�2.

ðCH3CH2Þ2OHþ �!nhn CH3CH2OH þ2 þ CH2QCH2 ð8Þ

Moore also observed some m/z 47 in their IRMPD experi-

ments on the proton-bound diethyl ether dimer which was

presumably due to sequential IRMPD.26

In previous experiments by Solca and Dopfer on the ethanol

proton-bound dimer in the 3400–3700 cm�1 range using a Nd/

YAG-pumped OPO laser only the m/z 47 photoproduct was

monitored.27 That in these experiments using an OPO, the

protonated ethanol photoproduct was observed is also evi-

dence that the higher energy photoproducts are observed for

high intensity lasers as opposed to the experiments conducted

with a low-power CO2 laser.17 The results of the present

experiments, however, imply that in order to get an accurate

IRMPD spectrum it might be necessary to monitor all photo-

products. Since the variation of the branching ratio with

intensity is small for the ethanol proton-bound dimer it is

expected that the effects on the spectra produced by Solca and

Dopfer27 experiments would be minimal.

Observed IRMPD photoproducts of the propanol proton-

bound dimer (m/z 121) include m/z 103 (protonated dipropyl

ether), m/z 61 (protonated propanol) and m/z 43 (protonated

propene). Protonated propene most likely occurs due to

IRMPD of protonated propanol. This was confirmed by

isolating protonated propanol and conducting the same

IRMPD experiments. The ratio of primary photoproducts of

the propanol proton-bound dimer, m/z 103 : m/z 61, is

virtually unvaried over the range of infrared shown in Fig.

7, unlike the other proton-bound dimers. As well, the average

ratio is about 1 : 10 for these products, showing even less

dehydration product (eqn (1)) than the previous two proton-

bound dimers, even though the potential energy surfaces are

similar, i.e. the enthalpy threshold for dehydration is 15.7 kJ

mol�1 less16 than that for dissociation into the neutral and

protonated alcohol. Note that the intensity of m/z 43 was

included in the intensity of m/z 61 in order to obtain the m/z

103 : m/z 61 ratio (Fig. 7).

Fig. 5 RRKM calculated rate constants as a function of internal

energy (top axis) and number of 1000 cm�1 photons absorbed (bottom

axis) for the methanol proton-bound dimer dissociating according to

eqn (1) (solid line) and eqn (2) (dashed line).

Fig. 6 Plot of the ion product intensity ratios (m/z 75 : m/z 47) vs.

radiation wavenumber for IRMPD of the ethanol proton-bound

dimer. Also shown is the IRMPD efficiency spectrum for proton-

bound dimer of ethanol. The arrows point toward the ordinates for

each plot.

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For methanol, the rate constants for the two pathways

become equal after the proton-bound dimer absorbs 16

photons (1.94 eV internal energy) and have a value of B1.3 �107 s�1. In experiments conducted with a low power CO2

laser,17,28 dissociation must be dominant over net29 IRMP

absorption which is why only the product with the lowest

energy requirement (SN2 pathway, reaction 1) is observed. In

the present experiments on the proton-bound methanol dimer,

both products are observed because a very high powered laser

is used so that IRMP absorption is competing with the

dissociation processes. Furthermore, when the molecule ab-

sorbs most strongly, net IRMP absorption is even faster such

that enough photons are absorbed to make reaction 2 more

competitive with reaction 1. For the ethanol and propanol

proton-bound dimers, the rate constants become equal at

higher internal energies, 2.10 eV and 2.28 eV, respectively,

where the rate constants are 6.3 � 104 and 1.0 � 105 s�1,

respectively, two orders of magnitude lower than the rate

constants for the proton-bound dimer. Since the IR absorp-

tion cross sections for all three proton-bound dimers are

predicted to be similar, the rate constant for IRMP absorption

is expected to be similar for all three proton-bound dimers. It

is therefore easy to postulate that the ethanol and propanol

proton-bound dimers will absorb more photons than the

methanol proton-bound dimer prior to dissociation, increas-

ing the internal energy to such a level that the rate constant for

the higher energy route, eqn (2), is greater making this simple

dissociation channel the dominant one. In fact, this explana-

tion is consistent with the observation of both reaction chan-

nels for the propanol proton-bound dimer being observed in

the CO2 laser IRMPD experiments by Bomse and Beau-

champ.17 Even with the low-power CO2 laser, the rate of

absorption is competitive with the dissociation rates, especially

for the larger proton-bound dimers.

3.2. IRMPD spectra

3.2.1 Proton-bound dimer of methanol. In Fig. 8 and Fig. 9

the IRMPD spectrum of the methanol proton-bound dimer is

presented (solid black trace). Also shown in Fig. 9 are the

computed infrared spectra for the two lowest energy structures

found at the B3LYP/6-31þG** level of theory. The computed

spectra shown are unscaled. Conformer MA is lower in energy

than MB by only 1.3 kJ mol�1. Both conformers are of C2

symmetry and their geometries are shown in Fig. 10.

The strongest absorption is centred at 866 cm�1 and is

attributed to the asymmetric O–H–O stretch. This strong band

is in the same vicinity as was observed for the homogenous

proton-bound dimers of ether previously.13,14 For both con-

formers, the O–H–O asymmetric stretch is predicted to occur

at 801 cm�1 (796 cm�1 for conformer MA) which is in general

agreement. The band observed at 982 cm�1 is most likely the

C–O stretching vibration. For conformer MA, the in-phase

(ip) and out of phase (op) C–O stretches are both predicted to

occur at 969 cm�1, the op stretch is predicted to be of

considerable intensity. For the conformer MB, the C–O

stretching vibrations are predicted to occur at 953 cm�1 (op)

and 991 cm�1 (ip). These predicted values are in good

Fig. 7 Plot of the ion product intensity ratios (m/z 103 : m/z 61) vs.

radiation wavenumber for IRMPD of the propanol proton-bound

dimer. Also shown is the IRMPD efficiency spectrum for proton-

bound dimer of propanol. The arrows point to the corresponding

ordinates.

Fig. 8 RRKM calculated rate constants as a function of internal

energy (top axis) and number of 1000 cm�1 photons absorbed (bottom

axis) for the ethanol (A) and propanol (B) proton-bound dimer

dissociating according to eqn (1) (solid line) and eqn (2) (dashed line).

960 | Phys. Chem. Chem. Phys., 2006, 8, 955–966 This journal is �c the Owner Societies 2006

agreement with the observed band. In the gas-phase30 the C–O

stretch for neutral methanol is centred at 1033.5 cm�1 (in-

dicated by the vertical dashed line in Fig. 9) and is predicted to

occur at 1045.9 cm�1 (unscaled) by the same level of theory as

used here for the proton-bound dimers. The red-shift of the

C–O stretch for the proton-bound dimer makes sense since the

C–O bond length is expected to increase and weaken upon

protonation. The calculations predict an increase in C–O bond

length in the proton-bound dimer to 1.47 A from the neutral

value of 1.426 A.

The broad band observed from 1070–1225 cm�1 is assigned

to the COH bend. The most intense modes predicted in this

region (Fig. 9) are COH bending motions (which are also

coupled to O–H–O stretching and CH3 rocking) occurring at

1071 and 1106 cm�1 for conformers MB andMA, respectively.

CH3 d-deformation is also predicted to occur in this region,

but with insignificant intensity. Neutral methanol also has

COH bend and CH3 wagging in this region, but they are

significantly weaker than the C–O stretch. That the COH bend

is higher in intensity (predicted and observed) for the methanol

proton-bound dimer is likely due to its being coupled to the

O–H–O asymmetric stretching motion. The next band

observed from 1330–1425 cm�1 can also be assigned to CH3

rocking coupled to COH bending, but for this mode the HCO

and HOC angles are in phase. Again, there is some O–H–O

asymmetric stretching associated with this which may account

for its intensity.

Finally, there is a broad feature observed between 1467 and

1634 cm�1. The strongest predicted bands in this region are

CH3 s-deformation (umbrella) modes at 1516 and 1543 cm�1,

for the conformer MB and MA, respectively, and the HOH

bending modes at 1570 and 1638 cm�1 for conformer MB and

1520 and 1639 cm�1 for conformer MA. For neutral metha-

nol, the umbrella mode is predicted to occur at 1484 cm�1

(experimental value, 1455 cm�1) but with insignificant inten-

sity. There is obviously no HOH bend for neutral methanol

and these are characteristic solely of the proton-bound alcohol

dimers. The observed infrared bands along with those calcu-

lated to be the most intense are summarized in Table 1.

In previous experiments24 using a CO2 laser, and based on

the extent of dissociation, a sharp feature was found to occur

at 1039.36 cm�1 (arrow in Fig. 9). This band actually occurs at

a position which is a minimum in IRMPD efficiency in the

present experiments. We do not have an explanation for this.

3.2.2 Proton-bound dimer of ethanol. The IRMPD effi-

ciency spectrum of the ethanol proton-bound dimer is shown

in Fig. 11. Also shown are calculated spectra of the three

lowest energy conformers identified. The lowest energy con-

former (EA) is of C2 symmetry as is the second lowest energy

conformer (EB). The third lowest energy conformer (EC) is of

C1 symmetry. In all, seven distinct conformers were found, all

within 7.1 kJ mol�1, and more may exist. In fact, nine

conformers of the neutral ethanol dimer were determined

computationally in a recent publication.31 In Fig. 12, only

the structures of the three lowest energy conformers men-

tioned above are outlined as a full study of the possible

conformers is beyond the scope of this work.

The O–H–O asymmetric stretch of the ethanol proton-

bound dimer is found at a somewhat lower wavenumber

position, 760 cm�1, than for the methanol proton-bound

dimer, 866 cm�1, a difference of 106 cm�1. The calculations

also predict a significant shift, 82 cm�1, which is in good

agreement with experiment (compare Fig. 9 and 11, as well as

Tables 1 and 2).

The CCO symmetric stretch is predicted to be coupled to

motion of the proton between the two monomers motion and

is seen to occur at 956 cm�1 based on comparison with the

predicted position and intensities as well as comparison with

the methanol proton-bound dimer spectrum. For comparison,

the symmetric C–C–O stretch of gas-phase neutral ethanol32

(also coupled to HCC bending) occurs at 884.6 cm�1. It is

difficult to compare the modes observed in neutral ethanol and

the proton-bound dimer due to the coupling of motion of the

Fig. 9 IRMPD efficiency spectrum for the methanol proton-bound

dimer (solid trace) as well as the calculated infrared spectrum (B3LYP/

6-31þG**, no scaling) for two conformers of the methanol proton-

bound dimer. The vertical dashed line shows the position of the C–O

stretch for neutral methanol and the arrow indicates the position of an

absorption determined using a CO2 laser (see text).24

Fig. 10 Structures of two conformers of the methanol proton-bound

dimers.

This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 955–966 | 961

central proton which obviously cannot be the case for neutral

ethanol.

The next band is seen to be somewhat resolved showing two

features centred at 1078 and 1137 cm�1 which are assigned to

asymmetric C–C–O stretching and CH2 rocking, respectively.

The asymmetric C–C–O stretch in gas-phase neutral ethanol

occurs at a similar position, 1089.2 cm�1. The next feature is

abroad, perhaps with some resolved features occurring be-

tween 1238–1660 cm�1. Rather than assigning any of the

resolved features, a number of modes predicted to be fairly

intense are listed in Table 2. This broad feature is most likely

due to CH2 and CH3 motions and quite obviously, HOH

bending as it is the strongest predicted absorption.

3.2.3 Proton-bound dimer of propanol. In Fig. 13 two

IRMPD spectra of the propanol proton-bound dimer (solid

line). For the less intense spectrum, an attenuator was used to

decrease the intensity of light impinging on the ions from the

CLIO FEL by a factor of three. That the relative intensities of

the bands remains similar in both spectra suggests that

IRMPD is not saturated at full laser power. The spectrum

of the propanol proton-bound dimer may, at first glance, seem

different from the methanol and ethanol proton-bound dimers

(see Fig. 14). The band(s) around 750 cm�1 are not nearly as

intense as observed for the other alcohols and there are more

bands resolved in the rest of the spectrum. Clearly the spec-

trum of the propanol proton-bound dimer is more complex

and will prove to be more difficult to interpret, which might be

expected since there are significantly more vibrational modes

compared to the ethanol and methanol proton-bound dimers.

There are also, however, many similarities between the three

spectra. While the methanol proton-bound dimer O–H–O

asymmetric stretch is significantly red-shifted, for the ethanol

and propanol proton-bound dimers this mode occurs in the

same vicinity, an observation which is reproduced by theory

(see Fig. 13 and Table 3). The bands observed with maxima at

737, 758(shoulder), and 802 cm�1 are likely all O–H–O asym-

metric stretching modes of different conformers. As well, the

band centred at 982 cm�1 is most likely the C–O stretching

vibration based on comparing the bands observed for the

other dimers and the calculated spectra (see Table 3 and Fig.

13). Finally the broad band between 1300 and 1600 cm�1,

Table 1 Summary of observed and B3LYP/6-31þG** predicted infrared bands for the methanol proton-bound dimer

Predicteda/cm�1

Observed/cm�1 Assignment Conformer MA Conformer MBNeutral methanolb

(gas-phase/cm�1)

866 O–H–O asym str 796 (2394) 801 (2449)982 CO str 969 (196) 991 (55) 1033.5 (vs)

1070–1225 COH bend/CH3 rock/O–H–O asym str 1106 (469) 1071 (766) 1060 (w)CH3 d-deformation/CH3 rock 1178 (77) 1177 (78) 1165 (w)

1330–1425 In phase COH bend/CH3 rock/ O–H–O asym str 1421 (505) 1352 (578) 1345 (s)1467–1634 CH3 s-deformation (umbrella) 1543 (760) 1516 (263) 1455 (m)

HOH bend 1520 (346) 1570 (73)1639 (69) 1638 (741)

a Values in parentheses are calculated intensities in km mol�1. b Ref. 30.

Fig. 11 IRMPD efficiency spectrum for the ethanol proton-bound

dimer (solid trace) as well as the calculated infrared spectrum (B3LYP/

6-31þG**, no scaling) for three conformers of the ethanol proton-

bound dimer.

Fig. 12 Structures of the three lowest-energy conformers found for

the ethanol proton-bound dimer.

962 | Phys. Chem. Chem. Phys., 2006, 8, 955–966 This journal is �c the Owner Societies 2006

perhaps with resolved features, is similar in shape and position

to the one observed for the ethanol proton bound dimer and

the two bands at higher wavenumber for the methanol proton-

bound dimer. In Table 3 the most intense bands predicted by

theory are presented for the three lowest-energy structures

(shown in Fig. 15).

3.2.4. Protonated propanol. Fig. 16 shows the experimental

spectrum for protonated propanol (dark solid line). Also

displayed in Fig. 16 are calculated spectra of four conformers

of protonated propanol whose wavenumber positions have

been scaled by 0.96. It was considered to be fair to use a scaling

factor in the case of protonated propanol since the modes in

this species are not unlike those for which scaling factors were

derived. This is not the case for the proton-bound dimers with

weaker hydrogen bonding which do not have enough data to

assign a scaling factor with confidence. The agreement be-

tween the experimental and calculated spectra is very good.

The assignments in Fig. 16 are based on comparing the

experimental and computed spectra. The strongest band ob-

served, centred at 1591 cm�1, is the HOH bend which is in

almost exactly the same position as the bend for gas-phase

water, 1595 cm�1.33 Obviously, in neutral ethanol this mode

does not exist and there is no absorption in this region of

neutral ethanol’s infrared spectrum.34 The C–O stretch of

neutral propanol occurs at 971 cm�1 in the gas phase.34 This

band is significantly shifted to a lower wavenumber, predicted

to occur at B650 cm�1 based on the calculated spectrum and

is not observed in these experiments due to the use of ZnSe

windows. The red shift of the C–O stretch upon protonation is

most likely to be due to the change in the C–O bond length

which is 1.57 A in protonated propanol and 1.43 A in neutral

propanol. A weaker band observed at 743 cm�1 is assigned to

the CO stretch/OH2 wag.

Table 2 Summary of observed and B3LYP/6-31þG** predicted infrared bands for the ethanol proton-bound dimer

Predicteda/cm�1

Observed/cm�1 Assignment Conformer EA Conformer EB Conformer EC Neutral ethanolb

760 O–H–O asym str 714 (2638) 714 (2788) 725 (211)956 CCO sym str/O–H–O str 867 (583) 870 (509) 874 (512) 884.6

1078 CCO asym str 1073 (173) 1059 (347) 1081 (209) 1089.21137 CH2 rocking 1164 (249) 1163 (301) 1156 (194) Not observed

1238–1660 OH bend/CH2 twist 1293 (198) 1283 (542) 1289 (183) 1241.31341 (676)

HOH bend 1427 (1201) 1565 (89) 1447 (1191)1636 (91) 1630 (668) 1645 (82)

CH2 wag/CH3 umbrella 1435 (311) 1435 (6) 1431 (88) 1393.71434 (36)

CH3 asym def 1501 (1.1) 1500 (10) 1503 (115) 1451.6CH2 deformation 1535 (201) 1523 (27) 1526 (160) 1490c

a Values in parentheses are calculated intensities in km mol�1. b Ref. 32. c In CD3CH2OH.

Fig. 13 IRMPD efficiency spectrum for the ethanol proton-bound

dimer (black trace) as well as the calculated infrared spectrum

(B3LYP/6-31þG**, no scaling) for three conformers of the propanol

proton-bound dimer. The colours of the calculated spectra correspond

to the coloured dots in Fig. 13.

Fig. 14 Comparison of the IRMPD efficiency spectrum for the

methanol, ethanol and propanol proton-bound dimers.

This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 955–966 | 963

3.2.5. Heterogeneous propanol/H2O proton-bound dimer.

The observed IRMPD fragments for m/z 79, the mixed

propanol/H2O proton-bound dimer, are m/z 61 (major pro-

duct), 43, 37 and 19. Presumably the primary IRMPD product

ions are protonated propanol, m/z 61, and the proton-bound

water dimer, m/z 37, which both have very similar reaction

enthalpies, differing by only 1 kJ mol�1, based on heats of

formation for H5O21, H2O, protonated propanol and propene

of 213, �242, 476 and 20.2 kJ mol�1, respectively.23 As

discussed above, m/z 43 is a product of subsequent IRMPD

of protonated propanol and m/z 19 is formed from subsequent

IRMPD of m/z 37. Both m/z 43 and m/z 19 were observed in

only very small amounts.

Table 3 Summary of B3LYP/6-31þG** predicted infrared bands for the propanol proton-bound dimer

Predicteda/cm�1

Observed/cm�1 Assignment Conformer PA Conformer PB Conformer PC

737 O–H–O asym str 710 (3160) 708 (3419) 734 (2444)758sh802852 CH2/CH3 twisting and rocking 904 (101) 906 (97) 901 (65)903982 CO str 968 (298) 958 (101) 972 (276)

1030–1140 CH2/CH3 wagging 1106 (224) 1088 (579) 1110 (249)1150–1280 CH2 twisting 1172 (229) 1170 (286) 1166 (186)

1256 (216) 1250 (529) 1256 (216)1290–1650 CH2 wagging 1324 (75) 1312 (460) 1326 (26)

1351 (228)CH3 umbrella 1415 (484) 1423 (87) 1409 (132)

1418 (92)COH bend/CH2 scissors 1442 (733) 1450 (1195)CH2 wag 1432 (347)CH3 scissors 1495 (125) 1503 (154)CH2 scissors 1511 (64) 1511 (59)

1523 (70) 1524 (95)HOH bend 1449 (94) 1573 (72) 1642 (82)

1620 (728)

a Values in parentheses are calculated intensities in km mol�1.

Fig. 15 Structures of the three lowest-energy conformers found for

the propanol proton-bound dimer.

Fig. 16 IRMPD efficiency spectrum of protonated propanol as well

as computed infrared spectra (B3LYP/6-31þG**, scale factor = 0.96)

of four conformers of protonated propanol.

964 | Phys. Chem. Chem. Phys., 2006, 8, 955–966 This journal is �c the Owner Societies 2006

The IRMPD efficiency spectrum is shown in Fig. 17 along

with the computed spectrum (dashed line) of the lowest-energy

isomer found for the proton-bound propanol/water dimer. It

is fairly obvious that the experimental and calculated spectra

are not in nearly as good agreement as for the homogenous

proton-bound dimers presented above and previously.4,14 The

C–O bond of this mixed dimer is predicted to be 1.52 A,

shorter than in protonated propanol, due to the ‘‘sharing’’ of

the proton between the two monomers. The C–O bond length

is also predicted to be longer than in the proton-bound dimer

of propanol since in the mixed proton-bound dimer, the

proton is bound more strongly to the oxygen than in the

homogeneous dimer. It is expected then, that the C–O stretch

would be to the blue of that observed in protonated propanol.

The band centred at 821 cm�1 is most likely the C–O stretch

which is predicted to occur at 823 cm�1. The broad band

centred at 1775 cm�1 is assigned to the HOH bending modes

(in phase and out of phase) of the two H2O moieties. The

HOH bend of the protonated water dimer was found to occur

at 1756 cm�1,4 in the same vicinity as that observed here for

the proton-bound propanol/water dimer. The calculated

bands are predicted to occur at 1615 and 1682 cm�1 in

disagreement by some 100 cm�1. A similar disagreement

between the level of theory used here and the experimental

spectra of homogenous proton bound dimers was seen pre-

viously.14 The O–H–O stretch is predicted to occur at 2357

cm�1, well outside the present observation window.

4. Conclusions

The infrared spectra of three homogeneous proton bound

dimers (methanol, ethanol, and propanol) were recorded and

bands were assigned based upon comparisons with the neutral

alcohol infrared spectra and computed infrared spectra. Simi-

larities in the three spectra, such as the position of the O–H–O

asymmetric stretch and the C–O stretch as well as the COH

bend are noted. The position of the O–H–O asymmetric

stretch for these proton-bound dimers is also similar in posi-

tion and prominence as for the ether proton-bound dimers

reported previously. The B3LYP/6-31þG** calculations pre-

sented are shown to adequately compare with the experimental

spectra. For the mixed water/propanol proton-bound dimer

assignments were made to the C–O stretch as well as the HOH

bend based on comparisons with the other spectra and that for

the water proton-bound dimer where the HOH bend was

previously assigned. Protonated propanol’s IRMPD spectrum

was also assigned.

Two primary IRMPD photoproducts were observed for

each of the alcohol proton bound dimers. These dissociation

routes differ in that the simple dissociation pathway, forming

protonated alcohol, is of higher energy but the entropy

requirement is lower compared with the SN2 process. The

ratio of these photoproducts were found to vary with the

frequency of the radiation impinging upon the ions. For

example, when the proton-bound dimer absorbs weakly a

larger amount of SN2 product, protonated ether and water,

are observed. When the proton-bound dimer absorbs more

strongly an increase in the simple dissociation product, pro-

tonated alcohol and neutral alcohol, is observed. With the aid

of RRKM calculations this frequency dependence of the

branching ratio is explained assuming that photon absorption

is faster than dissociation for these species and that only a few

photons extra are necessary to make the higher-energy dis-

sociation channel (simple cleavage) competitive with the lower

energy (SN2) reaction channel.

Acknowledgements

The CLIO team, especially J. M. Ortega and F. Glotin, is

gratefully acknowledged for their technical help. We also

thank the CNRS and the Universite de Paris XI for financial

support. The Natural Sciences and Engineering Research

Council of Canada is also acknowledged for financial support.

The Atlantic Computational Excellence Network (ACENet)

and the Memorial University of Newfoundland Advanced

Computation and Visualization Centre is gratefully acknowl-

edged for free computational resources. We would also like to

thank the referees for their helpful comments.

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